Intraspecific Variation in the Mammalian Gestation Period

Abstract

gestation period sheep bluetongue project Many summaries of the relationships between mean gestation period and other physiological and life history traits of terrestrial eutherian mammals have been published (Huggett and Widdas, 1951; Kihlstrim, 1972; Sacher and Staffeldt, 1974; Frazer and Huggett, 1974; Blueweiss et al., 1978; Case, 1978; Western, 1979; Tuomi, 1980; Zeveloff and Boyce, 1980; McNab, 1980; Lindstedt and Calder, 1981; Eisenberg, 1981; Millar, 1981), but data on intraspecific variation in gestation time have not been summarized and compared. Here I present a preliminary study of determinants of intraspecific variation in gestation time. The results may be of practical value in captive breeding programs and field studies of reproduction, and may provide insight into the relationships among breeding seasonality, gestation period, and the geographic distribution of mammalian species. Methods and results.-I gathered data on the mean, standard deviation (both in days), and coefficient of variation (CV = 100 SD/X) in gestation time from the literature on 48 mammalian species in captivity (Table 1). Only species with seven or more records of gestation length were included. When two or more sets of summary statistics on gestation time were available for a species, but individual observations were not reported, I used the data from the study with the largest sample size. My search for data ended when I judged that I had a representative set for the complete range of mean gestation periods found in eutherian species. Undoubtedly, I have overlooked some available data, especially for rodents. None of the species included is known usually to have delayed implantation, except for some rodents during lactation; where possible, I used data only for nonlactating females.